Lithography systems are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical lithography system includes an illumination source, a reticle stage assembly that positions a reticle, an optical assembly, a wafer stage assembly that positions a semiconductor wafer, a measurement system, and a control system. The measurement system constantly monitors the position of the reticle and the wafer, and the control system controls each stage assembly to constantly adjust the position of the reticle and the wafer. The features of the images transferred from the reticle onto the wafer are extremely small. Accordingly, the precise positioning of the wafer and the reticle is critical to the manufacturing of high quality wafers.
A typical measurement system includes one or more interferometers and/or one or more encoders. Certain types of interferometers and encoders utilize a light source that provides a pair of light beams that have a fixed frequency difference. Unfortunately, existing light sources have suffered from inefficiency, low power output, high costs and/or difficulty in fully separating the light beams. Accordingly, a system or assembly is desired which can avoid such drawbacks.